Bone the chicken, taking care not to slit or cut the skin.
Lay the boned chicken on a board, skin side downwards.
Peel and quarter the onion and chop finely, using the double-bladed chopping knife.
Saute in half the butter until tender.
Finely chop the ham and pork, using the double-bladed chopping knife, and add to the sauteed onion.
Coarsely chop the walnuts and mushrooms with the herbs and stir into the meat mixture.
Add the remaining ingredients and mix well.
Spread the stuffing over the chicken and shape into a roll, sewing up the edges of the chicken to enclose the stuffing.
To make the topping, make the bread into breadcrumbs, using the double-bladed chopping knife, then add a few sprigs of parsley and chop finely.
Stir in the Parmesan cheese and garlic.
Spread the remaining butter all over the chicken and then sprinkle with the topping.
Place in a baking tin with the tablespoon of oil.
Bake in a moderately hot oven (190C, 375 F, gas 5) for 1 -1 1/2 hours.